Iota (i) is the first letter of Iēsous (Ἰησοῦς), Greek for “Jesus”.
Chi (ch) is the first letter of Christos (Χριστός), Greek for “anointed”.
Theta (th) is the first letter of Theou (Θεοῦ), Greek for “God’s”, the genitive case of Θεóς, Theos, Greek for “God”.
Upsilon (u) is the first letter of huios (Υἱός), Greek for “Son”.
Sigma (s) is the first letter of sōtēr (Σωτήρ), Greek for “Savior”.
Historians say the 20th-century use of the ichthys motif is an adaptation based on an Early Christian symbol which included a small cross for the eye or the Greek letters “ΙΧΘΥΣ”.